1 Field of the Invention
This invention relates in general to the measurement of high-frequency electrical signals with a wide frequency band by electro-optical effect.
With the advent of photodetectors, photoconductive switches and other ultrafast electrical devices, the need arose for a measurement system capable of characterizing electrical signals with a resolution in the region of one picosecond.
2 Description of the Prior Art
In the past, such measurements were carried out by means of sampling oscilloscopes of resolution limited to approximately 25 picoseconds. Since then, the utilization of the electro-optical effect has introduced considerable developments in the characterization of an electric signal with resolutions in the region of one picosecond by a sampling technique. This new electro-optical technique for sampling an electric signal, e.g. by means of a Pockels cell lighted by ultra-short laser pulses, has been continually improved but has numerous drawbacks nonetheless.
Measurement systems based on time sampling require a very cumbersome (1.50 m) and costly short-pulse laser source. Moreover, to measure electric signals by a stroboscopic method as described in the article "40 GHz measurement on InP/air gap line by picosecond electro-optic sampling", by S. Loualiche, F. Clerot and G. Audibert, published in the ELECTRONICS LETTERS, Vol. 24, No. 4, p. 266-267, Feb. 15, 1990, the laser source used must be active mode-locked and the pulses generated then often have a period in excess of 30 picoseconds. It is then necessary to use a pulse compressor to effectively obtain pulse periods in the region of one picosecond. These systems are, however, difficult to adjust and introduce additional noise problems.
Furthermore, optical pulse sampling of an electrical signal by means of a stroboscopic method consisting of sampling the entire profile of said electrical signal requires a fluctuation of the width of the laser pulse (jitter) in the region of one picosecond. This, however, is not usually the case in mode-locked laser sources in which the light pulse width is in the region of 5 picoseconds. For measurement purposes, this jitter makes it necessary to use a phase stabilization loop which increases the complexity of the system and makes it more difficult (adjustment) and costly to use.